The invention relates to acoustic propagation from propulsion systems making use of ducted turbine engines, and more particularly it relates to an acoustic treatment cell for use in making up an acoustic treatment panel in order to attenuate the noise radiated by interaction between a rotor and its environment.
Conventionally, acoustic treatment for a turbojet, and more particularly treatment of the noise radiated by interaction between the rotor and its environment, is performed by using absorber panels arranged at the wetted surfaces of the ducts in which the soundwaves propagate. The term “wetted surfaces” is used to mean the surfaces in contact with a fluid flow. Such panels are made of sandwich type composite materials holding captive a honeycomb having absorbent properties that are obtained in part on the principle of Helmholtz resonators.
A Helmholtz resonator is constituted by a resonant cavity and a neck extending into the inside of the cavity from an orifice formed in a wall and enabling the resonant cavity to communicate with the surrounding medium in which the waves for attenuation are propagating. The neck thus provides communication between the ambient medium and the internal air cavity. Once the device has been optimized, the neck gives rise to a visco-thermal dissipation effect, which corresponds to rapid and alternating movement of soundwaves through the ends of the neck, thereby giving rise to dissipation by friction.
In conventional treatment technologies, the length of the neck is short compared with the height of the cavity. More precisely, in conventional technologies, the length of the neck is equal to the thickness of a wall made of composite sheet material (carbon+resin) constituting the wetted surface of the treatment, with the neck being obtained merely by perforating that wall. The operation of the Helmholtz resonator is optimized by dimensioning the air cavity so as to obtain the acoustic speed maximum at the neck. This optimization requires cavity height to be of the order of one-fourth of the wavelength of the main frequency that is to be treated. It also provides very advantageous properties concerning the width of the frequency band that is covered.
Nevertheless, present trends in optimizing propulsion systems are oriented towards reducing the number of blades and reducing the speed of rotation of the fan. This implies that the fan and the associated outlet guide vanes (OGVs) radiate acoustically at a lower frequency. The term “fan-OGV” is used to designate a system combining a rotor and a stator in the bypass stream. The rotor is referred to as the “fan”. The stator is made up of the outlet guide vanes. It would also be possible to use the term “rotor-stator”. The fan-OGV acoustic radiation frequency corresponds to the frequency of the acoustic radiation generated by the interaction between the blades of the rotor and the vanes of the stator, and also by the rotor itself.
Optimizing treatment panels then requires their thickness to be increased in order to be able to increase the height of the cavities and thus lower the frequency to which the resonant cavities in the panels are tuned. This makes the panels incompatible with the weight and size constraints associated with new ultra-high bypass ratio (UHBR) type architectures.
It is still possible to dimension the Helmholtz resonator in such a manner that it is effective at lower frequencies, while occupying little space radially, e.g. by acting both on the height of the neck and on the volume of the resonant cavity.
However, that type of dimensioning under the constraint of given small size, is achieved at the cost of the frequency band over which treatment is performed well being reduced drastically with decreasing frequency, as is shown in FIG. 1, which shows an acoustic attenuation curve plotting the effectiveness of treatment as a function of frequency for a conventional Helmholtz resonator as a continuous line and for a prior art Helmholtz resonator of dimensions that have been reduced as a dashed line.
FIG. 2 shows the results of measurements in the form of absorption coefficients concerning the low-frequency performance centered on 550 hertz (Hz) of a large neck structure having an overall thickness of 26 millimeters (mm) for a sound level of 140 decibels (dB), which measurements are plotted using white squares, and for a sound level of 156 dB, which measurements are plotted using black squares, in comparison with a conventional resonator based on a much thicker perforated sheet, plotted using a continuous line, which requires the use of a 150 mm cavity for performance that is equivalent in this frequency range.
This restriction concerning the width of the attenuation frequency band is very penalizing, since variation in the speed of the fan depending on the stage of flight gives rise to large changes in its sound emission frequency. As a result, acoustic treatment dimensioned in this way is effective at a single speed only.